This invention relates to nuclear magnetic resonance (NMR) apparatus. More specifically, this invention relates to monitoring lead wires having controlled impedance for use, for example, with NMR apparatus to monitor a physiological function, such as heart rate, of a live human or animal subject undergoing examination.
The nuclear magnetic resonance phenomenon has been utilized in the past in high resolution magnetic resonance spectroscopy instruments by structural chemists to analyze the structure of chemical compositions. More recently, NMR has been developed as a medical diagnostic modality having applications in imaging the anatomy, as well as performing in vivo, non-invasive spectroscopic analyses. As is now well known, the NMR phenomenon can be excited within a study subject positioned in a homogeneous polarizing magnetic field, having a strength of, for example, 1.5 T, by irradiating the object with radio-frequency (RF) energy at the Larmor frequency. In medical diagnostic applications, this is typically accomplished by positioning the patient to be examined in the field of an RF coil having cylindrical geometry, and energizing the RF coil with an RF power amplifier. Upon cessation of the RF excitation, the same or different RF coil is used to detect NMR signals emanating from the subject volume lying within the field of the RF coil. In the course of a complete NMR scan, a plurality of NMR signals are typically observed. The signals are used to derive NMR imaging or spectroscopic information about the subject studied.
Additionally, in typical medical imaging studies, pulsed linear magnetic field gradients are used to localize the signals to desired areas within the subject and to encode spatial information into the signals. In the course of an NMR examination, it is frequently desirable to apply pulsed magnetic field gradients in each of the X, Y, and Z directions of a conventional Cartesian coordinate system.
In some situations, it is desirable to monitor the subject during an NMR scan. This may be necessary for medical reasons in the case where the subject is an infirm patient. Another reason for monitoring the subject is to acquire signals in response to a change in a physiological characteristic of the subject and to use the signals to control some aspect of the scanning process. For example, electrocardiogram (ECG) signals can be used in a known manner as trigger signals in NMR cardiac gated studies. U.S. Pat. No. 4,413,233 entitled "Dynamic NMR Measurement," issued Nov. 1, 1983, discloses the use of trigger signals in gated NMR studies.
Typically, the gating NMR signals are acquired in a well-known manner using electrodes attached to the body of the subject. The electrodes are connected by means of electrode leads and additional cable and preamplifier as necessary to, for example, an ECG monitor. The lead wires and the cable can be modelled as a random transmission line with distributed inductance, capacitance, and resistance. The net impedance of such a transmission line varies as a function of lead wire and cable routing, placement and separation. Monitoring of the patient and acquisition of trigger signals of necessity takes place during an NMR scan so that the RF pulses and magnetic field gradient pulses can induce a current flowing in the circuit consisting of that part of the subject's body between the electrodes and the cable equivalent impedance. Thus, if the impedance at the electrode/subject body interface is higher than that of the conductors, resistive losses will occur at the interface, possibly resulting in undesirable heating of the body area in contact with the electrodes. Additionally, RF and magnetic field gradient pulses may introduce undesirable features (e.g., spikes) into the ECG signal waveform which can result in false triggering in gated studies.
It is, therefore, a principal object of the invention to provide apparatus to protect the subject of an NMR study from heating near the sight of monitoring lead contacts.
It is another object to eliminate the occurrence of false triggering resulting when radio-frequency energy is converted to low frequency energy in the monitoring lead wire.